Where it's found: Western yew is common along the West Coast, and eastern yew is common along the East Coast and parts of the central United States. Japanese yew is found throughout much of the United States.

What is the toxin and how does it work? Yew contains the toxic alkaloid taxine, which primarily affects the heart and respiratory system.

Threat to horses:

All parts of the plant except the fleshy portion of the red berries are poisonous.

The leaves are toxic even when dried.

Horses are often poisoned from ingesting discarded yew cuttings found in their pasture or from eating yew-made barn decorations such as wreaths and swags.

Ingestion of even a small amount results in signs. Depending on the individual horse, it takes anywhere from a few mouthfuls to 1 lb of leaves per 1,000 lbs of a horse’s body weight to cause death.

Signs: Sudden death is the most common sign. Other signs occur within an hour after ingestion. These signs include a slowed heart rate, difficulty breathing, trembling, lack of coordination, impaired movement, and inability to rise. Death generally follows 15 to 30 minutes after the onset of signs.

Treatment: Rapid development of illness and signs generally make treatment impossible. Veterinarians can use activated charcoal to decontaminate if done so early after ingestion. Other drugs such as atropine and lidocaine that focus on specific cardiac conduction abnormalities may be useful in horses that didn’t ingest a lethal dose.

Prognosis: Very poor to fatal. Sudden death is the normal outcome.

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Water hemlock (Cicuta spp.)

Where it's found: Water hemlock is often referred to as the most violently toxic plant in the United States. While found throughout the nation, the plant is less common in the Gulf Coast states. As its name suggests, water hemlock prefers wet areas, such as irrigation ditches, marshes, damp areas in pastures, and riverbanks.

What is the toxin and how does it work? Water hemlock contains the toxins cicutoxin and cicutol, which affect the neurons in the brain and central nervous system.

Threat to horses:

The plant’s roots contain the highest concentration of toxins, however horses rarely ingest these. It’s more likely that horses would eat the leaves and stems, which are also poisonous, when grazing in low-lying, moist areas.

The amount of toxins in the leaves and stems decreases as the plant ages and further decreases as the plant dries.

It takes about 0.2 to 2 lbs hemlock root per 1,000 lbs of a horse’s body weight to cause death.

Signs: These usually occur within an hour after ingestion. Because the toxins target the central nervous system, signs generally include agitation, nervousness, twitching, and seizures. Other signs include excessive salivation, dilated pupils, skeletal muscle weakness, cardiac abnormalities, difficult breathing, and death from respiratory paralysis.

Treatment: Veterinarians can use activated charcoal to decontaminate if the horse isn’t experiencing seizures and it can safely be accomplished. In the case of seizures, veterinarians can use pentobarbital to slow the activity of the horse’s brain and nervous system.

Prognosis: Very poor to fatal once signs have developed. Most horses are found dead within two to three hours after ingestion. Horses alive after eight hours of developing clinical signs are likely to recover, but usually have some residual cardiac and skeletal muscle damage.

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Poison hemlock (Conium maculatum)

Where it's found: Poison hemlock is found throughout the United States except for northern Minnesota, North and South Dakota, and eastern Montana. The plant grows in ditches, uncultivated areas, and near water such as swamps and lowlands. Poison hemlock looks similar to water hemlock. The only way to truly tell the difference between the two is by their roots.

What is the toxin and how does it work? Poison hemlock contains a number of alkaloid compounds including coniine, N-methyl coniine, and gamma-coniine. (Alkaloids are neurotoxins that affect both the central and peripheral nervous systems.)

Threat to horses:

The poisonous parts of the plant are the leaves, stems, and seeds. These parts increase in toxicity as the plant matures, especially with seeds, and decrease as the plant dies.

Most horses will avoid eating this plant; however, it can contaminate hay.

It takes about 2 to 8 lbs of plant material per 1,000 lbs of a horse’s body weight to cause death.

Signs: Signs generally occur within one to two hours after ingestion. Nervousness, tremors, twitching, lack of coordination, inability to rise, depression, and decreased heart and respiratory rates are common signs. Seizures occur in rare cases.

Treatment: There’s no specific treatment once signs appear, but supportive care is recommended. Veterinarians can use activated charcoal to decontaminate if the horse isn’t presenting with neurological signs and it can safely be accomplished.

Prognosis: Very poor to fatal. Many horses die from respiratory failure. However, those that survive the acute ingestion usually recover with no residual effects.

Where they're found: The plants are found throughout most of the United States; however, they're rarely found in Western Minnesota, North and South Dakota, and Eastern Montana. Most are weeds found in overgrazed pastures, meadows, and fields. A few varieties are grown as ornamental plants.

What is the toxin and how does it work? The plants contain the chemical ranunculin, which, when crushed or chewed, becomes the toxin protoanemonin. Protoanemonin is a bitter-tasting oil that irritates the mucous membranes of the gastrointestinal tract.

Threat to horses:

The plants generally don’t pose a serious threat because the toxin’s bitter taste and ability to cause mouth blisters limits ingestion. However, poisoning can occur in overgrazed pastures where there are little to no other plants for horses to consume.

The plants are most harmful when eaten fresh in the pasture or field.

The concentration of toxin varies depending on the species and growth stage of the plant. The flower part contains the highest amount of toxin.

Signs: These can occur as early as a few hours after ingestion or be delayed up to a day or two, depending on the amount ingested. Nose, lips, face, and skin may blister or swell after direct contact with plant. Blisters in the mouth, oropharynx, and esophagus also are common. Other signs include excessive salivation, an irritated gastrointestinal tract, colic, and bloody diarrhea. Tremors, seizures, and paralysis occur in rare cases.

Treatment: Removal of the horse from the pasture or field as soon as signs are noticed. Veterinarians may use activated charcoal to decontaminate; however, it should be used cautiously due to gastrointestinal tract irritation from the plants’ toxins. Supportive care including fluid therapy, gastrointestinal protectants, and analgesics are recommended.

Prognosis: Excellent with appropriate care. No residual effects have been noted.

Courtesy of Dr. Robert Poppenga, UC Davis

Yellow star thistle and Russian knapweed (Centauria spp.)

Where it's found:Centauria spp. is primarily found west of the Mississippi River between the Mexican and Canadian borders. It’s present as a weed in ditches, pastures, and fields.

What is the toxin and how does it work? The toxin is unknown but suspected to be repin, a sesquiterpene lactone. It affects the neurons in the brain and central nervous system that control chewing. It commonly results in nigropallidal encephalomalacia or the “chewing disease.”

Threat to horses:

Centauria spp. becomes dangerous when ingested in large amounts for a long period of time. Chronic ingestion of 60 percent to 200 percent of body weight for 30 to 60 days results in the onset of signs.

Centauria spp. also becomes a problem when it contaminates dry hay or overtakes a grazing pasture.

Only horses are affected by Centauria spp.; even mules and burros do not seem to develop signs when exposed to large amounts.

Signs: Signs don’t occur until the horse has ingested the plant for one to two months. At that time, the horse may experience drowsiness and frequent yawning. Horses will lose the ability to adequately chew food or drink fluids and will become anorexic and dehydrated. Other signs include involuntary facial tics, such as the twitching of the lips and tongue, and clenched facial muscles.

Treatment: There is no treatment once signs are present. The damage to the neurons is irreversible.

Prognosis: Very poor to fatal due to the permanent damage to neurons. Once a horse loses the ability to swallow, it will not return despite aggressive therapy.

Click here to see part one of this package, which features five more plants toxic to horses.

Dr. Hovda is director of veterinary services at SafetyCall International and Pet Poison Helpline in Bloomington, Minn.

Pet Poison Helpline is a service available 24 hours, seven days a week for pet owners and veterinary team members who require assistance treating a potentially poisoned pet and can provide treatment advice for poisoning cases of all species, including dogs, cats, birds, small mammals, large animals, and exotic species. As the most cost-effective option for animal poison control care, Pet Poison Helpline’s fee of $35 per incident includes follow-up consultation for the duration of the poisoning case. It is available in North America by calling 800-213-6680. Click here for additional information.